1. Field of the Invention
This invention relates to motion control systems. More particularly, this invention relates to computer-controllable machine tools.
2. Description of the Related Art
Automation has resulted in the development of motion controllers capable of signaling actuator devices to effect motion in linkages along a desired trajectory performing useful work. Motion controller permits increased speed and precision in performing a given task over manual operation. Robots and automated manufacturing equipment are examples of a few of the products that utilize motion control technology. Programming these devices are frequently accomplished by specifying the desired trajectory as a collection of line/arc segments, along with the desired velocity of each segment. With complex trajectories the velocities for each segment or group of segments is often a constant as optimization along each point in the trajectory would be very time consuming.
Most trajectory programmers have a fundamental understanding of the trade off between velocity and accuracy. They know that at higher velocities it becomes more difficult for the control to stay on the desired trajectory and thus trajectory programmers must make a trade off between the velocity and the precision of motion along the desired trajectory. These decisions are often based on the programmer's experience and result in an iterative programming process where the trajectory is executed and then modified to reduce the velocity in sections where an undesirable deviation from the desired trajectory is observed. Thus programmers control the deviation from the desired trajectory, and therefore the quality of the motion, by manipulating the velocity along the trajectory.
Nowhere is the attempt to maximize the velocity of motion control while minimizing the deviation from the desired trajectory more apparent than with motion control systems for manufacturing equipment often referred to as Computer Numerical Controllers (CNC). CNCs may be used to control manufacturing equipment such as lathes, grinders and mills. CNCs are computing devices adapted for the real-time control of machine tools. A numerical controller receives a set of coded instructions forming a part program. Part programs are frequently expressed in a standard G&M code language, or a close derivative of this language based on either the International Standards Organization (ISO) or the Electronics Industries Association (EIA) RS-274-D, using codes identified by letters such as G, M, F. The codes define a sequence of machining operations to control motion in the manufacture of a part. The numerical controller converts the codes to a series of electrical signals which control motors attached to a machine tool effecting the motion of the tool along the programmed trajectory.
A motion controller operating a milling machine is one example of CNC. Lathes grinders and coordinate measuring machines (CMMs) are other examples of manufacturing equipment which utilize a CNC for motion control. A 3-axis CNC milling machine has a head where a tool is mounted, a table movable relative to the tool in the X, Y plane. Motors control motion of the table in the X and Y directions and motion of tool in the Z direction, establishing an orthogonal X, Y, Z Cartesian coordinate system. Positional sensors (encoders or scales typically) provide feedback indicating the position of the tool with respect to the coordinate system of the milling machine. The CNC reads in a part program specifying a toolpath trajectory that the tool is to follow at a specified velocity or feedrate. The controller continuously compares the current tool position with the specified toolpath, and generates signals to control motors in such a way that the tool's actual trajectory matches the toolpath which is the desired trajectory as closely as possible while the tool moves along the toolpath at the desired velocity.
The deviation of the actual tool trajectory from the desired trajectory as expressed in the toolpath is called machining error. The machining error may be computed as the distance between the instantaneous tool position and the desired trajectory as specified by the toolpath. NC tolerance is defined to be the amount of the permitted machining error while machining. Motion controllers are expected to maintain good or tight NC tolerance. The machining error depends on many factors including the performance of the motion controller and the feedrate selected for traversing the trajectory during machining. In general, higher feedrates will result in larger machining errors.
Conventional part programs do not explicitly address NC tolerance issue and the machine tool operator—part programmer or machinist—must set feedrates to attempt to address this issue. In fact it can not be expressed using conventional NC programming languages, such as EIA RS-274-D, nor do existing motion controllers support the notion of constraining motion so that a NC tolerance specification is met. One of the operator's functions is to select feedrates that would result in acceptable part quality, while simultaneously achieving high metal removal rates. The selection of appropriate feed rates is based on the operator's experience and general rules of thumb may be obtained from numerous handbooks and charts (e.g., Machinery's Handbook, 24th edition, Industrial Press Inc., New York 1992). However, the figures from such documents, while perfectly feasible when used under the correct conditions, are frequently inappropriate when applied to specific machining situations. Published figures fail to account for local machining conditions such as sudden changes in the toolpath leaving optimization of the feedrate to the operator. It is difficult for operator to select feedrate values that achieve the desired part quality while maximizing the machine tool's productivity throughout the part program.